The condition of ametropia, in which images are not focused properly on the retina due to some refractive error in the eye, is pervasive in the general human population. Depending on the specific population considered, from 25% to 50% of persons have some degree of ametropia. The most common treatment for this condition is to place a corrective lens in the optical path, e.g. spectacles or contact lenses. Since conventional corrective measures have serious drawbacks, such as inconvenience and discomfort, there is substantial activity in the field of ophthalmology devoted to correcting ametropia by altering the optics of the eye itself. Most of this activity is concentrated on altering the cornea, which is the strong transparent membrane covering the central anterior portion of the eye.
The cornea contributes the greatest amount of refraction to the overall optical system of the eye due to its highly curved surface. Accordingly, small changes in corneal curvature can produce substantial changes in the focus of the eye. The cornea has five structural layers: epithelium, Bowman's membrane, stroma, Descemet's membrane, endothelium. The main load-bearing structures in the cornea are the uniform-diameter (about 30 nm) collagen fibrils within the many (several hundred) lamellae of the stroma. Much of the activity thus far has focused on altering the structure of the stroma within the cornea.
Several surgical methods have been used to alter the refractive properties of the cornea. A first surgical approach, Radial Keratotomy (RK), uses radial incisions to weaken the paracentral cornea in the circumferential direction by severing some collagen fibrils. Thus, this procedure can lead to circumferential expansion and consequent flattening of the optical zone in the central cornea. Another surgical approach removes layers of the stroma in specific patterns to achieve a "sculpting" of the corneal surface. Such removal can be by a keratome (knife) as in Automated Lamellar Keratoplasty (ALK), or by a laser, as in Photorefractive Keratectomy (PRK) and Laser in-situ Keratomileusis (LASIK). In PRK, material is ablated by laser directly from the anterior surface, thus destroying Bowman's layer and associated nerve endings. In LASIK, a flap of the cornea is first cut and folded back, so that only stromal layers are ablated. PRK, ALK, and LASIK all result in a thinned cornea. Thus, the remaining layers of the cornea must support the intraocular pressure. As a result, the optical effect of these surgeries depends in part on the stress-strain behavior of the remaining material.
Each of the surgical methods has serious side effects. The wounds from RK take several months to heal, and the resultant scarring tends to scatter light, leading to the perception of a "starburst" effect. RK, PRK, ALK, LASIK, and their variants are invasive or destructive, require time for wound healing, alter the natural structure of the cornea, offer limited predictability and stability, and often require similarly invasive corrections to maintain or improve the corrective alterations.
A non-surgical method of altering the shape of the cornea is through thermokeratoplasty, which heats selected locations of the cornea to at least the "collagen shrinkage temperature", causing the collagen fibrils to contract to about one-third their original length. Initially, heated wires were applied to the cornea, but it was found that the resulting collagen shrinkage was not stable, possibly due to the healing response of the tissue. Other methods of thermokeratoplasty have been proposed, such as using microwave radiation, radiofrequency antennae, ultrasound transducers, and lasers. The wide use of thermokeratoplasty was hindered, however, by the high temperature necessary for shrinkage of the collagen fibrils. Such high temperatures may permanently damage the more sensitive components of the cornea, especially the endothelium.
There remains a need for a safe, effective, non-invasive method of permanently altering the shape of the cornea to correct ametropia. There is also a need for an apparatus designed to practice this method of altering corneal shape.